CALIFORNIA BEARING RATIO CORRELATION WITH
SOIL INDEX PROPERTIES
MAK WAI KIN
A project report submitted in partial fulfillment of
the requirements for the award of the degree of
Master of Engineering (Civil – Geotechnics)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
MAY 2006
iii
To my beloved parents and sisters
iv
ACKNOWLEDGEMENT
I would like to take this opportunity to express my sincere appreciation to all
people and organization that had contributed towards the preparation of this final
project.
Firstly, I wish to thank my supervisor, Dr. Nurly Gofar, for spending her
precious time to supervise my works. I would not forget her invaluable guidance and
advices throughout this project.
Secondly, I am thankful to my company’s director and colleagues for their
support and understandings. Their very useful assistance while I am working allows
me to concentrate and complete the project within the specified time.
Last but not least, not to forget the full supports that has been given by my
parents during my study.
v
ABSTRACT
California Bearing Ratio (CBR) is a commonly used indirect method to
assess the stiffness modulus and shear strength of subgrade in pavement design
works, however; civil engineers always encounter difficulties in obtaining
representative CBR value for design of pavement. Over the years, many correlations
had been proposed by various researchers in which the soil index properties were
used to develop these correlations. A study was carried out to find the correlation
between CBR values with soil index properties that best suit the type of soils in
Malaysia. Analyses were carried out based on the published correlations and soil
data obtained from two highway project sites. Based on the results, it is observed
that the current published correlations are not suitable to be used in Malaysia. In
addition, no typical range could be found based on the soil index properties. A
correlation had been proposed in the study to predict the CBR values at top face of
the soil sample for fine-grained soil based on the soil data collated. These
correlations were developed based on the maximum dry density and optimum
moisture content.
vi
ABSTRAK
Nisbah Galas California (CBR) merupakan satu kaedah tidak langsung untuk
mengukur modulus kekerasan and kekuatan rich tanah bagi kerja-kerja rekabentuk
jalan raya berturap, tetapi; jurutera awam sentiasa menghadapi masalah untuk
mendapatkan nilai CBR yang boleh digunakan untuk rekabentuk. Tahun-tahun yang
lepas, banyak pertalian telah dicadangkan oleh banyak penyelidik dimana ciri-ciri
indeks tanah telah digunakan untuk mendapatkan pertalian ini. Satu penyelidikan
telah dijalankan untuk mendapatkan pertalian antara nilai CBR dengan ciri-ciri
indeks tanah yang boleh digunakan untuk jenis tanah di Malaysia. Analisis
berpandukan pertalian yang telah diterbitkan dan data tanah yang didapatkan dari dua
projek lebuhraya. Keputusan analisis menunjukkan pertalian yang telah diterbitkan
ini tidak sesuai digunakan di Malaysia. Tambahan lagi, tipikal had nilai CBR tidak
diperolehi berpandukan ciri-ciri indeks tanah. Satu pertalian baru telah dicadangkan
dalam penyelidikan ini untuk menganggar nilai CBR di muka atas sampel tanah
jelekit berpandukan data tanah yang dikumpul. Pertalian ini diterbitkan berpandukan
kepada ketumpatan kering maksimum dan kandungan lembapan optimum tanah.
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENTS iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES x
LIST OF FIGURES xi
LIST OF SYMBOLS xiv
LIST OF APPENDICES xv
1 INTRODUCTION 1
1.1 Background 1
1.2 Problem Statement 3
1.3 Aim and Objectives of Study 3
1.4 Scope of Study 4
2 LITERATURE REVIEW 5
2.1 California Bearing Ratio 5
2.1.1 Applications of California Bearing Ratio 6
2.1.2 Test Methods 7
2.1.2.1 In Situ Field Testing 8
2.1.2.2 Laboratory Testing 9
2.2 Soil Classification 11
2.2.1 Grain Size Distribution 12
2.2.2 Plasticity 15
viii
2.3 Correlations between CBR and Soil Classification 17
2.3.1 Design Manual for Roads and Bridges (1994) 17
2.3.2 Black (1962) 19
2.3.3 de Graft - Johnson and Bhatia (1969) 20
2.3.4 Agarwal and Ghanekar (1970) 21
2.3.5 National Cooperative Highway Research Program (2001)
22
2.4 Current Practice in Malaysia 23
3 METHODOLOGY 26
3.1 Introduction 26
3.2 Data Collection 28
3.2.1 Source of Data 28
3.2.2 Data Selection 29
3.3 Data Analysis 30
4 RESULTS AND DISCUSSIONS 32
4.1 Introduction 32
4.2 Particle Size Distribution 32
4.3 Relationship of CBR at Top Face and Bottom Face 35
4.4 Evaluation of Published Correlations 36
4.4.1 Coarse-grained Soil 37
4.4.2 Fine-grained Soil 39
4.4.2.1 NCHRP’s Correlation 39
4.4.2.2 Agarwal and Ghanekar’s Correlation 41
4.5 Typical Range of CBR Values 43
4.5.1 Coarse-grained Soil 44
4.5.2 Fine-grained Soil 45
4.6 Relationship of Maximum Dry Density with Optimum Moisture Content
48
4.7 Proposed Correlation for CBR Values 50
4.8 Discussion 52
4.8.1 Evaluation of Published Correlations 52
4.8.2 Typical Range of CBR Values 54
4.8.3 CBR Correlation with Soil Index Properties for Malaysia Soils
55
ix
5 CONCLUSIONS AND RECOMMENDATIONS 57
5.1 Conclusion 57
5.5 Recommendations for Future Research 59
REFERENCES 61
APPENDIX A – L 63 - 105
x
LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Definitions of soils classified by grading according to British Soil Classification System
13
2.2 Relationship of plasticity with liquid limit 16
2.3 Subgrade CBR estimation of British soils compacted at natural moisture content (The Highway Agency, 1994)
18
4.1 Particle size distribution test results for fine-grained soils 33
4.2 Particle size distribution test results for coarse-grained soils 34
xi
LIST OF FIGURES
FIGURE NO. TITLE PAGE
2.1 Dynamic cone penetrometer equipment 9
2.2 Test equipment for determination of CBR value in laboratory
11
2.3 Example of grading curves 14
2.4 Plasticity chart 16
2.5 Relationship between CBR and plasticity index at various liquidity index values
19
2.6 Correction of CBR values for partial saturation 19
2.7 Relationship between suitability index and soaked CBR values
20
2.8 Relationship between the ratio of maximum dry density to plasticity index and CBR for laterite-quartz gravels
21
3.1 Flowchart of the study 27
4.1 Relationship between CBRTOP and CBRBOTTOM values 35
4.2 Comparison of CBR with NCHRP’s line for coarse-grained soil
TOP 37
xii
4.3 Comparison of CBR with NCHRP’s line for coarse-grained soil
BOTTOM 38
4.4 Comparison of CBR with NCHRP’s line for fine-grained soil
TOP 40
4.5 Comparison of CBR with NCHRP’s line for fine-grained soil
BOTTOM 40
4.6 Relationship between CBRTOP and CBRA&G values 42
4.7 Relationship between CBRBOTTOM and CBRA&G values 42
4.8 Numbers of measurement of CBRTOP for coarse-grained soil
44
4.9 Numbers of measurement of CBRBOTTOM for coarse-grained soil
45
4.10 Numbers of measurement of CBRTOP for fine-grained soil
46
4.11 Numbers of measurement of CBRTOP(±3%) for fine-grained soil
47
4.12 Numbers of measurement of CBRBOTTOM for fine-grained soil
47
4.13 Numbers of measurement of CBRBOTTOM(±3%) for fine-grained soil
48
4.14 Relationship of maximum dry density with optimum moisture content
49
4.15 Proposed correlations for CBRTOP for fine-grained soil 51
xiii
LIST OF SYMBOLS
A - Percentage passing 2.4 mm BS sieve
CBR - California Bearing Ratio
CBRA&G - CBR value predicted by the Agarwal and Ghanekar’s correlation
CBRTOP - CBR value at top face of soil sample
CBRTOP(±3%) - Minimum CBRTOP within the range of ±3% of OMC
CBRBOTTOM - CBR value at bottom face of soil sample
CBRBOTTOM(±3%) - Minimum CBRBOTTOM within the range of ±3% of OMC
DCP - Dynamic Cone Penetrometer
D60 - Diameter at 60% passing from grain size distribution (mm)
LL - Liquid Limit
MDD - Maximum Dry Density
OMC - Optimum Moisture Content
PI - Plasticity Index
w - Percentage passing No.200 U.S. sieve (in decimal)
xiv
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Atterberg limits test results for coarse-grained soils 63
B Atterberg limits test results for fine-grained soils 64
C Compaction test results for coarse-grained soils 66
D Compaction test results for fine-grained soils 67
E Measured laboratory CBR values for coarse-grained soils
68
F Measured laboratory CBR values for fine-grained soils
70
G Measured soil index properties required for NCHRP’s correlations
72
H Estimated CBR values from NCHRP’s correlation for coarse-grained soils
74
I Estimated CBR values from NCHRP’s correlation for fine-grained soils
75
J Estimated CBR values based on Agarwal & Ghanekar’s Correlation
77
K Determination of the CBR value extracted from BS1377 Part 4:1990
78
xv
L Determination of the CBR value extracted from ASTM D 1883 - 92
98
2
CHAPTER 1
INTRODUCTION
1.1 Background
California Bearing Ratio (CBR) is frequently used index test value for civil
engineer particularly those in pavement construction to assess the stiffness modulus
and shear strength of subgrade. It is actually an indirect measure which represents
comparison of the strength of subgrade material to the strength of standard crushed
rock quoted in percentage values. The method was originally developed at
California Division of Highways in 1930s to provide an assessment of the relative
stability of fine crushed rock base material.
California Bearing Ratio is not something new to civil engineers in Malaysia
especially for those involved in road and airport pavement works. Usually, the CBR
values are used by pavement engineers to design the thickness of pavement that will
be laid on top of the subgrade. Subgrade that has lower CBR value will have thicker
pavement compared with the subgrade that has higher CBR value. In other words,
the design of pavement is very much dependent on the CBR value of subgrade.
Different soil types give different values of CBR although it is compacted at the
same amount of energy and rate of penetration.
Conventionally, CBR value can be measured directly in the laboratory test in
accordance with BS1377 on soil sample acquired from site. The soil sample will be
compacted as required in a standard mould and then a plunger is made to penetrate
the soil at a specified penetration rate. Load – deflection curve plotted from the
2
result of the penetration will be compared with that obtained from the standard crush
rock.
Apart from CBR test carried out in laboratory, engineer frequently conducts
indirect measurement of CBR value at project site. Dynamic Cone Penetrometer
(DCP) is a popular in-situ test method commonly used to estimate the in-situ CBR
value. However, the CBR value obtained from DCP test shall not be relied upon for
pavement design as it may represent unsoaked CBR value rather than soaked CBR
value which is required for design. Therefore, engineer is advised not to use the
CBR value obtained from DCP test for pavement design but only as a comparison
and estimation of CBR values that can be achieved by the subgrade.
DCP test although does not give exact soaked CBR value for design, it is
always proposed by engineers for subgrade assessment because it is an easy, cheap
and fast method compared with laboratory test. While laboratory test takes at least
four (4) days to measure the CBR value for each soil sample, DCP tests can give
immediate results of CBR values at various locations just in one day. Nevertheless,
it is still a good engineering practice that DCP test is being carried in a project as a
supplement to laboratory testing when assessing the shear strength and stiffness
modulus of subgrade.
A more reliable method of predicting CBR value of subgrade shall be
explored so that the engineers will have more options and confidence in obtaining a
representative soaked CBR value for pavement design.
One of the methods is by developing a correlation between CBR values with
soil index properties. There are few correlations that have been published by many
researchers since 1960s. In Malaysia, practising engineers seldom use these
correlations as it may be due to its unproven results on the Malaysia soils. Although
there are some researches had been carried out by our local universities, no extensive
data have been collated from a number of projects in Malaysia for verification
purposes.
3
1.2 Problem Statement
Civil engineers always encounter difficulties in obtaining representative
CBR value for design of pavement. Inadequate soil investigation data due to budget
constraint and poor planning of soil investigation works are regularly happened here
in Malaysia. In addition, laboratory CBR test required a relatively large soil sample
and is time consuming. Furthermore, the results sometimes are not accurate due to
the poor quality of handling and laboratory testing on the soil samples. Thus,
identification of factors that governs the CBR value such as index properties and
classification of the soil can be used as a base of the judgement on the validity of the
CBR values obtained in the field.
1.3 Aim and Objectives of Study
The aim of the study is to find correlation between CBR values with soil
index properties that best suit the type of soils in Malaysia. In order to achieve this
aim, three objectives have been identified for the study:
1. To evaluate published correlation for CBR value and the index properties of
soil based on collated data acquired from a number of projects in Malaysia.
2. To tabulate the CBR values obtained from collated soil samples and propose
a typical range of CBR values samples based on the soil index properties.
3. To obtain a correlation between CBR values with soil index properties that is
best suited for the type of soils in Malaysia.
4
1.4 Scope of Study
The study covers only the Malaysian practices in predicting CBR values for
pavement design. Site and laboratory tests will not be carried out thus all the soil
information and test results will be obtained from soil investigation contractors and
commercial laboratories.
The correlations to be reviewed and analysed in this study will be limited to
published correlations of CBR values with soil index properties that are generally
acceptable by engineers worldwide.
61
REFERENCES Agarwal, K.B. and Ghanekar, K.D. (1970). Prediction of CBR from Plasticity
Characteristics of Soil. Proceeding of 2nd South-east Asian Conference on Soil
Engineering, Singapore. June 11-15, 1970. Bangkok: Asian Institute of
Technology, 571-576.r
American Standard Test Method (1992). Standard Test Method for CBR (California
Bearing Ratio) of Laboratory-Compacted Soils. United States of America,
ASTM Designation D1883-92.
Black, W.P.M. (1962). A Method of Estimating the CBR of Cohesive Soils from
Plasticity Data. Geotechnique. Vol.12: 271 - 272.
British Standards Institution (1990). Methods of Test for Soils for Civil Engineering
Purposes. London, BS 1377.
British Standards Institution (1999). Code of Practice for Site Investigations.
London, BS 5950.
Carter, M. and Bentley, S. P. (1991). Correlations of Soil Properties. London:
Pentech Press.
de Graft - Johnson, J.W.S. and Bhatia, H.S. (1969). The Engineering Characteristics
of the Lateritic Gravels of Ghana. Proceedings of 7th Inernational Conference on
Soil Mechanics and Foundation Engineering, Mexico. August 28-29. Bangkok:
Asian Institute of Technology. Vol.2: 13 - 43.
62
National Cooperative Highway Research Program (2001) Guide for Mechanistic and
Empirical – Design for New and Rehabilitated Pavement Structures, Final
Document. In: Appendix CC-1: Correlation of CBR Values with Soil Index
Properties. West University Avenue Champaign, Illinois: Ara, Inc.
Steve, L. W., Richard, H. G. and Thomas, P. W. (1992) Description and Applications
of Dual Mass Dynamic Penetrometer. Washington, DC: US Army Corps of
Engineers.
Terzaghi, K., Peck, R.B. and Mesri, G. (1996) Soil Mechanics in Engineering
Practice. 3rd ed. United States of America: John Wiley & Sons, Inc.
The Highway Agency (1994) Design Manual for Roads and Bridges. In: Volume 7:
Section 2 Part 2 HD 25/94. London: Stationery Ltd.
